1. Field of the Invention
This invention relates generally to apparatus for removal of metals from wastewater, and more particularly, it relates to an improved wastewater treatment system and method for automatically controlling of chemical feed of organic sulfide products in wastewater containing metal ions so as to produce metal ion precipitation and removal of metals by sedimentation.
2. Description of the Prior Art
As is generally known, Oxidation-Reduction Potential electrodes are currently used to automate chemical feed control in continuous metal ion precipitation processes using alkyl dithiocarbamate (DTC), precipitant, or other organic sulfides. While Oxidation-Reduction Potential electrodes have been shown to respond to the presence of metal ions such as Cu(II), this electrode also responds to a myriad of electrochemically-active components that may be present in such wastewater, thus the electrode is non-specific. In the presence of an interference such as bleach, for instance, the Oxidation-Reduction Potential electrode is an unreliable sensor for precipitant feed control.
The only known literature or patent reference that describes a process that could be considered to slightly or remotely resemble that of the present invention, is U.S. Pat. No. 4,465,963 to M. F. Wemhoff. The disclosure of the Wemhoff patent is directed toward a system for the removal of metals from wastewater by chemical precipitation and filtration.
In the Wemhoff patent, the system 10 in FIG. 1 includes a holding tank 12 for storing untreated wastewater, the wastewater having metals dissolved therein. The system 10 processes the wastewater in the holding tank 12 in a batch-wise operation. The system further includes a reaction tank 14 which is connected by a fill line 16 to the holding tank. The reaction tank also receives a precipitating reagent from a tank 54 and a pH balancing reagent from a tank 50. A filter means 24 is coupled to the outlet of the reaction tank to receive liquid and precipitated metals therefrom. A manifold 30 connected to the outlet of the filter means is coupled to the inlet of the reaction tank 14.
An ion specific electrode 34 is disposed in the manifold 30, which is responsive to the free ion content of the wastewater, and is utilized to determine when the precipitation process carried on in the reaction tank 14 has been completed. A pH specific electrode 36 is also disposed in the manifold 30, which is responsive to the pH of the wastewater, and is utilized to control the pH thereof so as to prevent generating of toxic gas. A first valve means 32 controls the flow of filtrate through the manifold 30. A second valve means 40 controls the flow of filtrate through a drain line 38 which receives the fully treated effluent.
A microprocessor controller 46 is responsive to the ion specific and pH specific electrodes 34 and 36 and controls both the first and second valve means 32 and 34. Initially, the controller 46 causes the second valve means 40 to be closed and the first valve means to be opened so as to permit recirculation of treated wastewater from the output of the filter means 24 through the manifold 30 to the reaction tank 14. When the free ion content of the treated wastewater changes to a predetermined value as determined by the ion specific electrode 34, the microprocessor controller 46 causes the first valve means 32 to close and the second valve means 40 to open allowing the flow of effluent from the filter means 24 to the drain line 38. If the holding tank is not empty, the system allows the next batch of wastewater to be treated to be passed into the reaction tank 14.
The system in the Wemhoff patent is thus responsive only to the presence of excess sulfide which does not always allow for accurate detection and resulting precipitant feed because such ion specific electrodes are known to plateau under precipitant overfeed conditions such that excess precipitant overfeed may occur without viable detection. Excess precipitant overfeed is a concern since dithiocarbamates are also biocides.
The present invention of a wastewater treatment system and method represents an improvement of the aforementioned '593 patent and includes a solid-state sulfide ion-selective electrode for monitoring a first setpoint value corresponding to a condition of precipitant underfeed and a second setpoint value corresponding to a condition of slight precipitant overfeed. The sulfide ion-selective electrode is responding to the metal ions in the wastewater during the underfeed condition and is responding to the slight excess of precipitant during the overfeed condition so as to cause an electrochemical potential (mV) between the sulfide electrode and a reference electrode.